Two-section type rock core pressure flooding experimental device and method based on CT (Computed Tomography) scanning

Abstract

The invention discloses a two-section type rock core pressure flooding experiment device and method based on CT scanning. The device comprises a liquid supply system, a temperature control system, a two-section pressure control type core holder, a pressure loading system, a CT scanner and a metering device, the liquid supply system comprises a constant-speed constant-pressure pump and three intermediate containers, and the constant-speed constant-pressure pump is connected with the injection end of the two-section pressure control type core holder after being connected with the intermediate containers through a four-way valve; a core sample is arranged in the two-section pressure control type core holder, a front-end confining pressure injection hole and a rear-end confining pressure injection hole are formed in the side wall of the two-section pressure control type core holder, correspondingly connected with a confining pressure servo pump and a manual high-pressure pump and used for applying different confining pressures to the front section and the rear section of the core sample, and the outflow end of the two-section pressure control type core holder is connected with a metering device through a discharging pipeline. And a back-pressure valve and an outlet control valve which are communicated with the manual high-pressure pump connecting pipeline are arranged. The method truly restores the oil field on-site pressure driving construction crack forming process, and provides a basis for research on the change of cracks in a reservoir before and after pressure driving.

Description

Technical Field

[0001] The invention relates to the technical field of pressure-driven development of low-permeability oil reservoirs, and in particular to a two-stage core pressure-driven experimental device and method based on CT scanning. Background Art

[0002] In view of the current situation that water injection wells cannot inject water and oil production wells cannot produce oil in low permeability oil reservoirs, technical personnel in the field have carried out a series of high-pressure water injection pilot tests and achieved certain results. Based on field practice, technical personnel in the field have proposed pressure-driven technology, combining hydraulic fracturing equipment with water injection development, and using large-displacement high-pressure pumping equipment to perform short-term high-pressure water injection above the formation fracture pressure, so that the rocks around the wellbore break and form cracks, thereby increasing the reservoir pressure and seepage capacity in a relatively short period of time, and increasing the oil well production capacity and reservoir recovery rate.

[0003] Based on the knowledge of field practice, it can be inferred that a large number of cracks are generated in the reservoir rocks around the water injection wells during the pressure drive process of low permeability reservoirs. However, it is difficult to directly obtain the changes in the internal structure of the reservoir during the field construction of the oil field, which leads to unclear understanding of the distribution morphology and pressure drive mechanism of pressure drive cracks. When studying the pressure drive process in indoor physical simulation experiments, the working characteristic of the universal core holder is that the injection pressure must be less than the confining pressure. If the confining pressure is higher than the injection pressure, the core will not be able to quickly inject fluid to produce pressure fractures. Therefore, it is urgent to propose a two-stage core pressure drive experimental device and method based on CT scanning to accurately obtain the crack distribution morphology and pressure drive fracture pressure during the pressure drive process of low permeability and tight reservoir cores, and to study the extension and expansion of cracks during the pressure drive process of low permeability cores. Summary of the invention

[0004] The present invention aims to solve the above problems and proposes a two-stage core pressure drive experimental device and method based on CT scanning, which realizes the real simulation of the internal fracture generation process of the reservoir during the on-site pressure drive construction in the oil field, provides technical support for studying the changes of fractures in the reservoir before and after pressure drive, and is conducive to guiding the pressure drive development of the reservoir.

[0005] To achieve the above object, the present invention adopts the following technical solution:

[0006] A two-stage core pressure drive experimental device based on CT scanning, comprising a fluid supply system, a temperature control system, a two-stage pressure-controlled core holder, a pressure loading system, a CT scanner and a metering device;

[0007] The fluid supply system comprises a constant speed and constant pressure pump and three intermediate containers, the three intermediate containers are respectively a formation water intermediate container, an oil intermediate container and a pressure drive liquid intermediate container, the constant speed and constant pressure pump is respectively connected to the high-pressure injection pipelines of the formation water intermediate container, the oil intermediate container and the pressure drive liquid intermediate container through a four-way valve, and is used to inject fluid into the two-stage pressure controlled core clamp; the outflow ends of the formation water intermediate container, the oil intermediate container and the pressure drive liquid intermediate container are all connected to high-pressure liquid outlet pipelines, each high-pressure liquid outlet pipeline is connected to the injection pipeline arranged at the injection end of the two-stage pressure controlled core clamp through a six-way valve, the outflow end of the two-stage pressure controlled core clamp is connected to the metering device through a discharge pipeline, and the discharge pipeline is provided with a back pressure valve and an outlet control valve;

[0008] The two-stage pressure-controlled core clamp is used to clamp the core sample during pressure displacement; the CT scanner is matched with the two-stage pressure-controlled core clamp to monitor the fracture morphology of the core sample in real time during the pressure displacement process;

[0009] The two-stage pressure-controlled core holder and the three intermediate containers in the fluid supply system are respectively connected to a temperature control system for controlling the temperature of the fluid injected into the core sample;

[0010] The pressure loading system includes a confining pressure servo pump and a manual high-pressure pump, wherein the confining pressure servo pump is connected to the two-section pressure-controlled core clamp through a front-end pressure injection pipeline, and is used to apply confining pressure to the front end of the core sample in the two-section pressure-controlled core clamp; the manual high-pressure pump is connected to the two-section pressure-controlled core clamp through a rear-end pressure injection pipeline, and is used to apply confining pressure to the rear end of the core sample in the two-section pressure-controlled core clamp; a three-way valve is arranged on the rear-end pressure injection pipeline, and the back-pressure valve is connected to the three-way valve through a back-pressure valve connecting pipeline.

[0011] Preferably, the two-stage pressure-controlled core clamp comprises a barrel, a front sealing cover and a rear sealing cover, the barrel has a core sample built in it, and the two ends of the core sample are sealed by the front sealing cover and the rear sealing cover respectively; one end of the core sample is provided with a water injection hole, and the other end is provided with a water outlet hole, the core experimental jacket is provided with a rubber sleeve, and the rubber sleeve is provided with a partition device, which separates the annular cavity enclosed by the rubber sleeve and the inner wall of the barrel of the two-stage pressure-controlled core clamp into a front end cavity and a rear end cavity;

[0012] The outer wall of the cylinder is provided with a front confining pressure injection hole and a rear confining pressure injection hole; the front confining pressure injection hole is connected to the front cavity and connected to the front pressure injection pipeline; the rear confining pressure injection hole is connected to the rear cavity and connected to the rear pressure injection pipeline;

[0013] The front sealing cover and the rear sealing cover are both provided with insertion holes; the outflow end of the injection pipe passes through the insertion hole and the water injection hole of the front sealing cover in sequence, and is inserted into the core sample for injecting fluid into the core sample; the inflow end of the discharge pipe passes through the insertion hole and the water outlet hole of the rear sealing cover in sequence, and is inserted into the core sample for discharging the fluid in the core sample.

[0014] Preferably, an injection pressure gauge is provided on the six-way valve; a front end confining pressure gauge is provided on the front end pressure injection pipeline near the confining pressure servo pump; and a rear end confining pressure gauge is provided on the rear end pressure injection pipeline near the manual high-pressure pump.

[0015] Preferably, the temperature control system is configured as a constant temperature box.

[0016] Preferably, the confining pressure servo pump is connected to an injection pressure gauge for obtaining the injection pressure of the fluid in the core sample in real time.

[0017] A two-stage core pressure drive experimental method based on CT scanning, using the two-stage core pressure drive experimental device based on CT scanning as described above, measures the pressure drive fracture pressure and crack morphology of the core sample based on the constant speed method, comprising the following steps:

[0018] Step 1, prepare a core sample, drill a circular groove for simulating fracturing perforation at the center of the side wall of the core sample, clean the surface of the core sample, dry it, evacuate it, saturate it with formation water, and then place it in a two-stage pressure-controlled core holder;

[0019] Step 2, injecting formation water, oil and pressure-driven fluid into the formation water intermediate container, the oil intermediate container and the pressure-driven fluid intermediate container respectively;

[0020] Step 3, according to the preset front end confining pressure value, the confining pressure servo pump is controlled to apply confining pressure to the front end cavity of the two-stage pressure controlled core clamp, and according to the preset rear end confining pressure value, the manual high-pressure pump is controlled to apply confining pressure to the rear end cavity of the two-stage pressure controlled core clamp. At this time, the front end confining pressure value is equal to the rear end confining pressure value. At the same time, the temperature control system is controlled according to the formation temperature of the reservoir to be simulated, so that the fluid temperature in the intermediate container and the temperature of the core sample in the two-stage pressure controlled core clamp are constant at the formation temperature, and then the four-way valve, the six-way valve and the outlet control valve are adjusted, and the formation water in the formation water intermediate container is injected into the two-stage pressure controlled core clamp by a constant temperature and pressure pump, and the core sample is displaced by formation water, and the formation water injection pressure is obtained according to the indication of the injection pressure gauge, and the water output of the discharge pipe is measured by a metering device. After the formation water injection pressure and the water output are stable, the initial water permeability of the core is calculated;

[0021] Step 4, adjusting the four-way valve and the six-way valve, controlling the constant speed and constant pressure pump to inject the oil in the oil intermediate container into the two-stage pressure-controlled core holder at a preset flow rate, displacing the core sample with oil, and measuring the water output of the discharge pipe with a metering device. When the core sample reaches the initial oil-containing state, the water output of the discharge pipe no longer increases. The core sample in the initial oil-containing state is CT scanned with a CT scanner to obtain a three-dimensional core data body corresponding to the initial oil-containing state and construct a three-dimensional digital core to obtain the microscopic pore structure of the core sample in the initial oil-containing state;

[0022] Step 5, control the confining pressure servo pump and the manual high-pressure pump, apply the rear confining pressure to the rear cavity of the two-stage pressure-controlled core clamp by the manual high-pressure pump according to the formation pressure of the reservoir to be simulated, and apply the front confining pressure to the front cavity of the two-stage pressure-controlled core clamp by the confining pressure servo pump, so that the front confining pressure is greater than the rear confining pressure, adjust the four-way valve and the six-way valve, control the constant speed constant pressure pump so that the injection pressure of the fluid in the two-stage pressure-controlled core clamp gradually increases to the rear confining pressure value, continue to control the constant speed constant pressure pump to increase the injection pressure of the fluid in the two-stage pressure-controlled core clamp and ensure that the front confining pressure is always greater than the injection pressure of the fluid in the two-stage pressure-controlled core clamp during the fluid injection process, inject the pressure-driven fluid in the pressure-driven fluid intermediate container into the two-stage pressure-controlled core clamp, use the pressure-driven fluid to displace the core sample, observe the injection pressure gauge, and use the metering device to measure the oil output of the discharge pipeline to obtain the pressure-driven fluid injection pressure and recovery rate change curve over time;

[0023] Step 6, after the injection pressure indication is stable, the constant temperature and constant pressure pump is controlled to gradually increase the injection speed of the pressure-driven fluid according to the preset pressure gradient, and the injection pressure change curve under different flow rates is obtained. The indication of the injection pressure gauge is observed until a sudden pressure drop occurs. At this time, cracks are generated in the core sample. The critical fracture pressure of the pressure-driven fluid is determined according to the indication of the injection pressure gauge when the pressure drops suddenly. The core sample after the cracks are generated is scanned by a CT scanner to obtain a three-dimensional data body of the core after the cracks are generated and a three-dimensional digital core is constructed to obtain the microscopic pore structure and crack morphology of the core sample after the cracks are generated.

[0024] A two-stage core pressure drive experimental method based on CT scanning, using the two-stage core pressure drive experimental device based on CT scanning as described above, simulating the pressure drive fracture expansion law and the pressure drive enhanced oil recovery effect based on the constant pressure method, comprising the following steps:

[0025] Step 1, prepare two identical core samples, drill a circular groove for simulating fracturing perforation at the center of the side wall of the core sample, clean the surface of the core sample, dry it, evacuate it, saturate it with formation water, and then randomly select one core sample and place it in a two-stage pressure-controlled core holder;

[0026] Step 2, injecting formation water, oil and pressure-driven fluid into the formation water intermediate container, the oil intermediate container and the pressure-driven fluid intermediate container respectively;

[0027] Step 3, according to the preset front end confining pressure value, the confining pressure servo pump is controlled to apply confining pressure to the front end cavity of the two-stage pressure controlled core clamp, and according to the preset rear end confining pressure value, the manual high-pressure pump is controlled to apply confining pressure to the rear end cavity of the two-stage pressure controlled core clamp. At this time, the front end confining pressure value is equal to the rear end confining pressure value. At the same time, the temperature control system is controlled according to the formation temperature of the reservoir to be simulated, so that the fluid temperature in the intermediate container and the temperature of the core sample in the two-stage pressure controlled core clamp are constant at the formation temperature, and then the four-way valve, the six-way valve and the outlet control valve are adjusted, and the formation water in the formation water intermediate container is injected into the two-stage pressure controlled core clamp by a constant temperature and pressure pump, and the core sample is displaced by formation water, and the formation water injection pressure is obtained according to the indication of the injection pressure gauge, and the water output of the discharge pipe is measured by a metering device. After the formation water injection pressure and the water output are stable, the initial water permeability of the core is calculated;

[0028] Step 4, adjusting the four-way valve and the six-way valve, controlling the constant speed and constant pressure pump to inject the oil in the oil intermediate container into the two-stage pressure-controlled core holder at a preset flow rate, displacing the core sample with oil, and measuring the water output of the discharge pipe with a metering device. When the core sample reaches the initial oil-containing state, the water output of the discharge pipe no longer increases. The core sample in the initial oil-containing state is CT scanned with a CT scanner to obtain a three-dimensional core data body corresponding to the initial oil-containing state and construct a three-dimensional digital core to obtain the microscopic pore structure of the core sample in the initial oil-containing state;

[0029] Step 5, control the confining pressure servo pump and the manual high-pressure pump, apply the rear confining pressure to the rear cavity of the two-stage pressure-controlled core clamp by the manual high-pressure pump according to the formation pressure of the reservoir to be simulated, and apply the front confining pressure to the front cavity of the two-stage pressure-controlled core clamp by the confining pressure servo pump, so that the front confining pressure is greater than the rear confining pressure, adjust the four-way valve and the six-way valve, control the constant speed constant pressure pump so that the injection pressure of the fluid in the two-stage pressure-controlled core clamp gradually increases to the rear confining pressure value, continue to control the constant speed constant pressure pump to increase the injection pressure of the fluid in the two-stage pressure-controlled core clamp and ensure that the front confining pressure is always greater than the injection pressure of the fluid in the two-stage pressure-controlled core clamp during the fluid injection process, inject the pressure-driven fluid in the pressure-driven fluid intermediate container into the two-stage pressure-controlled core clamp, use the pressure-driven fluid to displace the core sample, observe the injection pressure gauge, and use the metering device to measure the oil output of the discharge pipeline to obtain the pressure-driven fluid injection pressure and recovery rate change curve over time;

[0030] Step 6, after the injection pressure indication is stable, the constant temperature and constant pressure pump is controlled to gradually increase the injection speed of the pressure-driven fluid according to the preset pressure gradient, and the injection pressure change curves under different flow rates are obtained, and the indication of the injection pressure gauge is observed until a sudden pressure drop occurs. At this time, cracks are generated in the core sample, and the pressure-driven critical fracture pressure is determined according to the indication of the injection pressure gauge when the pressure drops suddenly, and the core sample after the cracks are generated is CT scanned by a CT scanner to obtain a three-dimensional data body of the core after the cracks are generated and construct a three-dimensional digital core to obtain the microscopic pore structure and crack morphology of the core sample after the cracks are generated;

[0031] Step 7, using another core sample to replace the core sample in the two-stage pressure-controlled core holder, repeating steps 3 and 4, and obtaining the microscopic pore structure of the replaced core sample in the initial oil-containing state;

[0032] Step 8, according to the indication of the injection pressure gauge, control the confining pressure servo pump to apply a confining pressure value higher than the injection pressure of the fluid in the two-stage pressure-controlled core holder to the cavity at the front end of the two-stage pressure-controlled core holder, set a constant temperature and pressure pump according to the pressure-driven critical fracture pressure determined in step 6, control the constant temperature and pressure pump to inject the pressure-driven fluid in the pressure-driven fluid intermediate container into the core under the pressure-driven critical fracture pressure condition, use the pressure-driven fluid to displace the core sample, and use a metering device to measure the oil output of the discharge pipeline and the pressure-driven fluid flow rate;

[0033] Step 9, after a preset amount of pressure-driven fluid is injected into the two-stage pressure-controlled core holder, a CT scanner is used to perform a CT scan on the core sample after the pressure-driven fluid is injected to obtain a three-dimensional digital core and pore structure corresponding to the core sample in the current state;

[0034] Step 10, after the injection of the pressure-driven fluid is completed, adjust the four-way valve and the six-way valve, control the constant temperature and pressure pump to inject the formation water in the formation water intermediate container into the two-stage pressure-controlled core clamp at a constant speed, and use the formation water to displace the core sample until the oil output measured by the metering device no longer changes, then end the formation water displacement of the core sample, and use the CT scanner to perform a CT scan on the core sample again to obtain the three-dimensional digital core and pore structure corresponding to the core sample after the pressure drive is completed.

[0035] Beneficial technical effects brought by the present invention:

[0036] (1) The present invention proposes a two-stage core pressure-driven experimental device based on CT scanning. By setting a front cavity and a rear cavity in a two-stage pressure-controlled core clamp, different confining pressures are applied to the front and rear halves of the core sample, so that the confining pressure at the injection end of the core sample is greater than the injection pressure of the fluid, effectively ensuring that the pressure-driven fluid is injected into the core sample at a high pressure, avoiding the problem that the injection pressure of the pressure-driven fluid of the rock sample is higher than the confining pressure it is subjected to during the experiment, causing the core clamp to leak and lead to experimental failure, and truly restoring the pressure-driven process of the rock in the reservoir.

[0037] (2) The present invention also proposes a two-stage core pressure-drive experimental method based on CT scanning, which uses a core sample to simulate the pressure-drive process of rocks in the reservoir in conjunction with a two-stage core pressure-drive experimental device based on CT scanning. During the experiment, a pressure difference is formed inside and outside the core sample to cause the core to break, and the fracture morphology and pore structure of the core sample are obtained in conjunction with CT scanning, thereby achieving accurate simulation of the pressure-drive fracture pressure, fracture morphology, pressure-drive fracture expansion law and pressure-drive enhanced oil recovery effect, which is beneficial to studying the extension and expansion of fractures inside the rock after pressure-drive, and provides a basis for improving the seepage capacity of the reservoir and increasing the production capacity and recovery rate of the oil well. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a structural schematic diagram of a two-stage core pressure drive experimental device based on CT scanning of the present invention.

[0039] Figure 2 It is a schematic diagram of the two-stage pressure-controlled core holder of the present invention.

[0040] Figure 3 It is a schematic diagram of the water injection hole and the water outlet hole of the core sample of the present invention.

[0041] In the figure, 1. constant speed and constant pressure pump, 2. four-way valve, 3. formation water intermediate container, 4. oil intermediate container, 5. pressure drive liquid intermediate container, 6. six-way valve, 7a. injection pressure gauge, 7b. front confining pressure gauge, 7c. rear confining pressure gauge, 8. two-stage pressure-controlled core holder, 801. cylinder, 802. front sealing cover, 803. rear sealing cover, 804. rubber sleeve, 805. separator, 806. front cavity, 807. rear cavity, 808. front confining pressure injection hole, 809. rear confining pressure injection hole, 9. core sample, 901. water injection hole, 902. water outlet hole, 10. CT scanner, 11. confining pressure servo pump, 12. back pressure valve, 13. outlet control valve, 14. metering device, 15. manual high-pressure pump, 16. three-way valve, 17. temperature control system. DETAILED DESCRIPTION

[0042] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0043] Example 1

[0044] This embodiment discloses a two-stage core pressure drive experimental device based on CT scanning, such as Figure 1 As shown, it includes a fluid supply system, a temperature control system, a two-stage pressure-controlled core holder, a pressure loading system, a CT scanner and a metering device.

[0045] The fluid supply system comprises a constant speed and constant pressure pump 1 and three intermediate containers, wherein the three intermediate containers are respectively a formation water intermediate container 3, an oil intermediate container 4 and a pressure drive liquid intermediate container 5, the inflow ends of the formation water intermediate container, the oil intermediate container and the pressure drive liquid intermediate container are all provided with high pressure injection pipelines, and the outflow ends are all provided with high pressure outlet pipelines, the constant speed and constant pressure pump is respectively connected to the high pressure injection pipelines of the formation water intermediate container, the oil intermediate container and the pressure drive liquid intermediate container through a four-way valve 2, and is used to inject the fluid in the intermediate container into the two-stage pressure controlled core clamp 8 for core displacement; the outflow end of the formation water intermediate container, the oil intermediate container and the pressure drive liquid intermediate container are provided with high pressure injection pipelines, and the outflow end is provided with high pressure outlet pipelines, and the constant speed and constant pressure pump is respectively connected to the high pressure injection pipelines of the formation water intermediate container, the oil intermediate container and the pressure drive liquid intermediate container through a four-way valve 2, and is used to inject the fluid in the intermediate container into the two-stage pressure controlled core clamp 8 for core displacement; The high-pressure liquid outlet pipes at the outlet end and the outlet end of the pressure-driven liquid intermediate container are both connected to the six-way valve 6, and the six-way valve is connected to the injection pipe arranged at the injection end of the two-stage pressure-controlled core clamp. The six-way valve can be controlled to control the connection between the outflow end of the formation water intermediate container, the outflow end of the oil intermediate container and the pressure-driven liquid intermediate container and the two-stage pressure-controlled core clamp, and fluid is injected into the two-stage pressure-controlled core clamp. The outflow end of the two-stage pressure-controlled core clamp is connected to the metering device 14 through a discharge pipe, and a back pressure valve 12 and an outlet control valve 13 are arranged on the discharge pipe. The metering device is used to obtain the discharge amount of the liquid in the discharge pipe.

[0046] The two-stage pressure-controlled core clamp 8 is used for clamping the core sample 9 by pressure displacement. The CT scanner 10 is provided in conjunction with the two-stage pressure-controlled core clamp to monitor the fracture morphology of the core sample in real time during the pressure-driven process. The two-stage pressure-controlled core clamp 8 and the three intermediate containers in the fluid supply system are respectively connected to the temperature control system 17 to control the temperature of the fluid injected into the core sample. In this embodiment, the temperature control system 17 adopts a thermostat, and the temperature of the thermostat is set according to the actual formation temperature of the reservoir to be simulated, so as to truly restore the temperature environment of the reservoir where the core sample is located.

[0047] The two-stage pressure-controlled core holder 8 is as follows Figure 2 As shown, it includes a cylinder 801, a front sealing cover 802 and a rear sealing cover 803. A prefabricated core sample 9 is placed in the cylinder, and the two ends of the core sample 9 are sealed by the front sealing cover 802 and the rear sealing cover 803 respectively; one end of the core sample 9 is provided with a water injection hole 901 with a diameter of 6 mm, and the other end is provided with a water outlet hole 902 with a diameter of 6 mm, as shown in FIG. Figure 3As shown, the core experiment jacket is provided with a rubber sleeve 804, and a partition device 805 is provided on the rubber sleeve. The partition device 805 is an annular protrusion, which divides the annular cavity surrounded by the rubber sleeve, the inner wall of the two-stage pressure-controlled core clamp cylinder, the front sealing cover and the rear sealing cover into a front end cavity 806 and a rear end cavity 807, so that different confining pressures can be applied to the front and rear sections of the core sample.

[0048] A front confining pressure injection hole 808 and a rear confining pressure injection hole 809 are provided on the outer wall of the cylinder, which are used to apply confining pressure to the core sample; the front confining pressure injection hole 808 is connected to the front cavity 806 and is connected to the front pressure injection pipeline, and the front confining pressure is applied to the front half of the core sample through the front pressure injection pipeline; the rear confining pressure injection hole 809 is connected to the rear cavity 807 and is connected to the rear pressure injection pipeline, and the front confining pressure is applied to the rear half of the core sample through the rear pressure injection pipeline.

[0049] The front sealing cover and the rear sealing cover are both provided with insertion holes; the outflow end of the injection pipe passes through the insertion hole and the water injection hole of the front sealing cover in sequence, and is inserted into the core sample for injecting fluid into the core sample; the inflow end of the discharge pipe passes through the insertion hole and the water outlet hole of the rear sealing cover in sequence, and is inserted into the core sample for discharging the fluid in the core sample.

[0050] In this embodiment, a separation device is used to divide the space between the rubber sleeve and the inner wall of the two-section pressure-controlled core clamp into a front cavity and a rear cavity, so that different confining pressures can be applied to the front and rear halves of the core sample, ensuring that the confining pressure at the injection end of the core sample is higher than the injection pressure of the fluid, so that the displacement fluid can be injected into the core sample at high pressure.

[0051] The pressure loading system includes a confining pressure servo pump 11 and a manual high-pressure pump 15, wherein the confining pressure servo pump 11 is connected to the front end confining pressure injection hole of the two-section pressure-controlled core clamp through a front end pressure injection pipeline, and is used to apply confining pressure to the front end of the core sample in the two-section pressure-controlled core clamp; the manual high-pressure pump 15 is connected to the rear end confining pressure injection hole of the two-section pressure-controlled core clamp through a rear end pressure injection pipeline, and is used to apply confining pressure to the rear end of the core sample in the two-section pressure-controlled core clamp; a three-way valve 16 is arranged on the rear end pressure injection pipeline, and the back pressure valve 12 is connected to the three-way valve through a back pressure valve connecting pipeline, and the back pressure valve 12 and the three-way valve 16 are always in an open state, so that the rear end pressure injection pipeline, the back pressure valve connecting pipeline and the discharge pipeline are connected.

[0052] In this embodiment, multiple pressure gauges are provided for real-time pressure monitoring. The six-way valve is provided with an injection pressure gauge 7a, which is connected to the confining pressure servo pump 11, and is used to monitor the injection pressure of the fluid in the core sample in real time and transmit it to the confining pressure servo pump in real time. The front-end pressure injection pipeline is provided with a front-end confining pressure gauge 7b near the confining pressure servo pump, which is used to obtain the front-end confining pressure value of the core sample in real time; the rear-end pressure injection pipeline is provided with a rear-end confining pressure gauge 7c near the manual high-pressure pump, which is used to obtain the rear-end confining pressure value of the core sample in real time.

[0053] Example 2

[0054] In this embodiment, a two-stage core pressure drive experimental method based on CT scanning is proposed. The two-stage core pressure drive experimental device based on CT scanning as described in Example 1 is used to measure the pressure drive fracture pressure and crack morphology of the core sample based on the constant speed method, including the following steps:

[0055] Step 1, prepare a core sample, drill a circular groove for simulating fracturing perforation at the center of the side wall of the core sample, clean the surface of the core sample, dry it, evacuate it, saturate it with formation water, and place it in a two-stage pressure-controlled core holder.

[0056] In this embodiment, rock samples are drilled at representative strata in the reservoir to make cylindrical cores, and a circular groove for simulating fracturing perforation is drilled at the center of the side wall of the core sample to obtain the core sample. After the surface of the core sample is cleaned, dried, vacuumed, and saturated with formation water, the core sample is used as a reservoir pressure drive model, and the core sample is used to replace the rock in the reservoir to be simulated for experiment.

[0057] Step 2: obtain formation water and oil from the reservoir to be simulated, prepare pressure-driven fluid in advance, and inject the formation water, oil and pressure-driven fluid into the formation water intermediate container, the oil intermediate container and the pressure-driven fluid intermediate container respectively.

[0058] Step 3, according to the preset front-end confining pressure value, control the confining pressure servo pump to apply confining pressure to the front-end cavity of the two-stage pressure-controlled core clamp, and according to the preset rear-end confining pressure value, control the manual high-pressure pump to apply confining pressure to the rear-end cavity of the two-stage pressure-controlled core clamp. At this time, the front-end confining pressure value is equal to the rear-end confining pressure value. At the same time, the temperature control system is controlled according to the formation temperature of the reservoir to be simulated, so that the fluid temperature in the intermediate container and the temperature of the core sample in the two-stage pressure-controlled core clamp are kept constant at the formation temperature, and then adjust the four-way valve and the six-way valve so that the constant temperature and pressure pump, the formation water intermediate container and the two-stage pressure-controlled core clamp are The two-stage pressure-controlled core holders are connected by pipelines, and the oil intermediate container and the pressure-driven liquid intermediate container are not connected to the two-stage pressure-controlled core holder, that is, only the formation water in the formation water intermediate container can be injected into the two-stage pressure-controlled core holder by a constant temperature and pressure pump, and the core samples in the two-stage pressure-controlled core holders are displaced by the formation water. The formation water injection pressure is obtained according to the indication of the injection pressure gauge, and the water output of the discharge pipe is measured by a metering device. After the formation water injection pressure and the water output are stable, the initial water permeability of the core is calculated using the Darcy formula. The Darcy formula is a prior art in this field and will not be repeated here.

[0059] Step 4, adjust the four-way valve and the six-way valve so that the constant temperature and pressure pump, the oil intermediate container and the two-stage pressure-controlled core clamp are connected through the pipeline, and the formation water intermediate container and the pressure-driven liquid intermediate container are not connected to the two-stage pressure-controlled core clamp, that is, only the constant temperature and pressure pump can be used to inject the oil in the oil intermediate container into the two-stage pressure-controlled core clamp, control the constant speed and pressure pump to inject the oil in the oil intermediate container into the two-stage pressure-controlled core clamp at a constant flow rate of 0.08ml / min, use oil to displace the core sample, use a metering device to measure the water output of the discharge pipe, when the core sample reaches the initial oil-containing state, the water output of the discharge pipe no longer increases, use a CT scanner to perform CT scanning on the core sample in the initial oil-containing state, obtain the corresponding three-dimensional core data body in the initial oil-containing state and construct a three-dimensional digital core, and obtain the microscopic pore structure of the core sample in the initial oil-containing state.

[0060] Step 5, control the confining pressure servo pump and the manual high-pressure pump, and use the manual high-pressure pump to apply the rear-end confining pressure to the rear-end cavity of the two-stage pressure-controlled core clamp according to the formation pressure of the reservoir to be simulated, and use the confining pressure servo pump to apply the front-end confining pressure to the front-end cavity of the two-stage pressure-controlled core clamp, so that the front-end confining pressure is greater than the rear-end confining pressure, adjust the four-way valve and the six-way valve, so that the constant temperature and pressure pump, the pressure drive fluid intermediate container and the two-stage pressure-controlled core clamp are connected through the pipeline, and the formation water intermediate container and the oil intermediate container are not connected to the two-stage pressure-controlled core clamp, and control the constant speed and pressure pump to inject the fluid in the two-stage pressure-controlled core clamp After the pressure gradually increases to the rear end confining pressure value, continue to control the constant speed constant pressure pump to increase the injection pressure of the fluid in the two-stage pressure controlled core holder and ensure that the front end confining pressure is 2MPa higher than the injection pressure of the fluid in the two-stage pressure controlled core holder (i.e., the pressure of pressure gauge 7a) during the fluid injection process. Then control the constant speed constant pressure pump to inject the pressure driving fluid in the pressure driving fluid intermediate container into the two-stage pressure controlled core holder at a constant flow rate of 0.1ml / min, use the pressure driving fluid to displace the core sample, observe the injection pressure gauge, and use the metering device to measure the oil output of the discharge pipe to obtain the pressure driving fluid injection pressure and recovery rate change curve with time.

[0061] Step 6, after the injection pressure indication is stable, the constant temperature and constant pressure pump is controlled to gradually increase the injection speed of the pressure-driven fluid according to the preset pressure gradient of 0.1 ml / min, and the injection pressure change curve under different flow rates is obtained. The indication of the injection pressure gauge is observed until a sudden pressure drop occurs. When a sudden pressure drop occurs, it indicates that cracks have occurred in the core sample. The critical fracture pressure of the pressure-driven fluid is determined according to the indication of the injection pressure gauge when the pressure drops suddenly. A CT scanner is used to perform a CT scan on the core sample after the cracks are generated, and a three-dimensional data body of the core after the cracks are generated is obtained, and a three-dimensional digital core is constructed to obtain the microscopic pore structure and crack morphology of the core sample after the cracks are generated.

[0062] It can be seen that compared with the pressure-driven experiment using the traditional core clamp, the real fracturing effect can only be simulated when the injection pressure of the pressure-driven fluid is higher than the confining pressure of the core sample. In the actual application process, the injection pressure of the pressure-driven fluid in the rock sample is higher than the confining pressure it is subjected to, which is easy to cause leakage of the core clamp, thereby causing the experiment to fail. In this embodiment, the two-stage core pressure-driven experimental device based on CT scanning sets a front cavity and a rear cavity in the two-stage pressure-controlled core clamp to apply different confining pressures to the front and rear halves of the core sample, so that the confining pressure at the injection end of the core sample is greater than the injection pressure of the fluid, effectively ensuring that the pressure-driven fluid is injected into the core sample at a high pressure, forming the necessary pressure difference inside and outside the core sample to achieve the core fracture condition, and truly restoring the pressure-driven process of the rock in the reservoir, and realizing the accurate simulation of the pressure-driven fracture pressure and crack morphology of the rock in the reservoir.

[0063] Example 3

[0064] This embodiment proposes a two-stage core pressure drive experimental method based on CT scanning, using the two-stage core pressure drive experimental device based on CT scanning as described in Example 1, and simulating the pressure drive fracture expansion law and pressure drive enhanced oil recovery effect based on the constant pressure method, including the following steps:

[0065] Step 1: prepare two identical core samples, drill a circular groove at the center of the side wall of the core sample to simulate fracturing perforation, clean the surface of the core sample, dry it, evacuate it, and saturate it with formation water, then randomly select one core sample and place it in a two-stage pressure-controlled core holder.

[0066] In this embodiment, rock samples are drilled at representative strata in the reservoir to make cylindrical cores, and a circular groove for simulating fracturing perforation is drilled at the center of the side wall of the core sample to obtain the core sample. After the surface of the core sample is cleaned, dried, vacuumed, and saturated with formation water, the core sample is used as a reservoir pressure drive model to perform experiments using the core sample instead of the rock in the reservoir to be simulated.

[0067] Step 2: obtain formation water and oil from the reservoir to be simulated, prepare pressure-driven fluid in advance, and inject the formation water, oil and pressure-driven fluid into the formation water intermediate container, the oil intermediate container and the pressure-driven fluid intermediate container respectively.

[0068] Step 3, according to the preset front-end confining pressure value, control the confining pressure servo pump to apply confining pressure to the front-end cavity of the two-stage pressure-controlled core clamp, and according to the preset rear-end confining pressure value, control the manual high-pressure pump to apply confining pressure to the rear-end cavity of the two-stage pressure-controlled core clamp. At this time, the front-end confining pressure value is equal to the rear-end confining pressure value. At the same time, the temperature control system is controlled according to the formation temperature of the reservoir to be simulated, so that the fluid temperature in the intermediate container and the temperature of the core sample in the two-stage pressure-controlled core clamp are kept constant at the formation temperature, and then adjust the four-way valve and the six-way valve so that the constant temperature and pressure pump, the formation water intermediate container and the two-stage pressure-controlled core clamp are The two-stage pressure-controlled core holders are connected by pipelines, and the oil intermediate container and the pressure-driven liquid intermediate container are not connected to the two-stage pressure-controlled core holder, that is, only the formation water in the formation water intermediate container can be injected into the two-stage pressure-controlled core holder by a constant temperature and pressure pump, and the core samples in the two-stage pressure-controlled core holders are displaced by the formation water. The formation water injection pressure is obtained according to the indication of the injection pressure gauge, and the water output of the discharge pipe is measured by a metering device. After the formation water injection pressure and the water output are stable, the initial water permeability of the core is calculated using the Darcy formula. The Darcy formula is a prior art in this field and will not be repeated here.

[0069] Step 4, adjust the four-way valve and the six-way valve so that the constant temperature and pressure pump, the oil intermediate container and the two-stage pressure-controlled core clamp are connected through the pipeline, and the formation water intermediate container and the pressure-driven liquid intermediate container are not connected to the two-stage pressure-controlled core clamp, that is, only the constant temperature and pressure pump can be used to inject the oil in the oil intermediate container into the two-stage pressure-controlled core clamp, control the constant speed and pressure pump to inject the oil in the oil intermediate container into the two-stage pressure-controlled core clamp at a constant flow rate of 0.08ml / min, use oil to displace the core sample, use a metering device to measure the water output of the discharge pipe, when the core sample reaches the initial oil-containing state, the water output of the discharge pipe no longer increases, use a CT scanner to perform CT scanning on the core sample in the initial oil-containing state, obtain the corresponding three-dimensional core data body in the initial oil-containing state and construct a three-dimensional digital core, and obtain the microscopic pore structure of the core sample in the initial oil-containing state.

[0070] Step 5, control the confining pressure servo pump and the manual high-pressure pump, and use the manual high-pressure pump to apply the rear-end confining pressure to the rear-end cavity of the two-stage pressure-controlled core clamp according to the formation pressure of the reservoir to be simulated, and use the confining pressure servo pump to apply the front-end confining pressure to the front-end cavity of the two-stage pressure-controlled core clamp, so that the front-end confining pressure is greater than the rear-end confining pressure, adjust the four-way valve and the six-way valve, so that the constant temperature and pressure pump, the pressure drive fluid intermediate container and the two-stage pressure-controlled core clamp are connected through the pipeline, and the formation water intermediate container and the oil intermediate container are not connected to the two-stage pressure-controlled core clamp, and control the constant speed and pressure pump to inject the fluid in the two-stage pressure-controlled core clamp After the pressure gradually increases to the rear end confining pressure value, continue to control the constant speed constant pressure pump to increase the injection pressure of the fluid in the two-stage pressure controlled core holder and ensure that the front end confining pressure is 2MPa higher than the injection pressure of the fluid in the two-stage pressure controlled core holder (i.e., the pressure of pressure gauge 7a) during the fluid injection process. Then control the constant speed constant pressure pump to inject the pressure driving fluid in the pressure driving fluid intermediate container into the two-stage pressure controlled core holder at a constant flow rate of 0.1ml / min, use the pressure driving fluid to displace the core sample, observe the injection pressure gauge, and use the metering device to measure the oil output of the discharge pipe to obtain the pressure driving fluid injection pressure and recovery rate change curve with time.

[0071] Step 6, after the injection pressure indication is stable, the constant temperature and constant pressure pump is controlled to gradually increase the injection speed of the pressure-driven fluid according to the preset pressure gradient of 0.1 ml / min, and the injection pressure change curve under different flow rates is obtained. The indication of the injection pressure gauge is observed until a sudden pressure drop occurs. When a sudden pressure drop occurs, it indicates that cracks have occurred in the core sample. The critical fracture pressure of the pressure-driven fluid is determined according to the indication of the injection pressure gauge when the pressure drops suddenly. A CT scanner is used to perform a CT scan on the core sample after the cracks are generated, and a three-dimensional data body of the core after the cracks are generated is obtained, and a three-dimensional digital core is constructed to obtain the microscopic pore structure and crack morphology of the core sample after the cracks are generated.

[0072] Step 7, using another core sample to replace the core sample in the two-stage pressure-controlled core holder, repeating steps 3 and 4, and obtaining the microscopic pore structure of the replaced core sample in the initial oil-containing state;

[0073] Step 8, according to the indication of the injection pressure gauge, control the confining pressure servo pump to apply a confining pressure value higher than the injection pressure of the fluid in the two-stage pressure-controlled core holder to the front cavity of the two-stage pressure-controlled core holder. At this time, the front confining pressure value of the core sample is 2MPa higher than the injection pressure of the fluid in the two-stage pressure-controlled core holder (i.e., the pressure of the pressure gauge 7a). According to the pressure-driven critical fracture pressure determined in step 6, a constant temperature and pressure pump is set, and the constant temperature and pressure pump is controlled to inject the pressure-driven fluid in the pressure-driven fluid intermediate container into the core under the pressure-driven critical fracture pressure condition, and the core sample is displaced by the pressure-driven fluid, and the oil output and the pressure-driven fluid flow rate of the discharge pipeline are measured by a metering device.

[0074] Step 9, after a preset amount of pressure-driven fluid is injected into the two-stage pressure-controlled core clamp, the preset amount in this embodiment is set according to the pore volume of the core sample, specifically 0.5 times the pore volume of the core sample, and a CT scanner is used to perform a CT scan on the core sample after the pressure-driven fluid is injected to obtain a three-dimensional digital core and pore structure corresponding to the core sample in the current state.

[0075] Step 10, after the injection of the pressure-driven fluid is completed, the four-way valve and the six-way valve are adjusted so that the constant temperature and pressure pump, the formation water intermediate container and the two-stage pressure-controlled core clamp are connected through the pipeline, and the oil intermediate container and the pressure-driven fluid intermediate container are not connected to the two-stage pressure-controlled core clamp, that is, only the constant temperature and pressure pump can be used to inject the formation water in the formation water intermediate container into the two-stage pressure-controlled core clamp at a constant flow rate of 0.08 ml / min, and the formation water is used to displace the core sample in the two-stage pressure-controlled core clamp until the oil output measured by the metering device no longer changes, and the formation water displacement of the core sample is terminated, and the core sample is scanned again by the CT scanner to obtain the three-dimensional digital core and pore structure corresponding to the core sample after the pressure drive is completed.

[0076] It can be seen that compared with the pressure-driven experiment using the traditional core clamp, the real fracturing effect can only be simulated when the injection pressure of the pressure-driven fluid is higher than the confining pressure of the core sample. In the actual application process, the injection pressure of the pressure-driven fluid in the rock sample is higher than the confining pressure it is subjected to, which is easy to cause leakage of the core clamp, thereby causing the experiment to fail. In this embodiment, the two-stage core pressure-driven experimental device based on CT scanning sets a front cavity and a rear cavity in the two-stage pressure-controlled core clamp to apply different confining pressures to the front and rear halves of the core sample, so that the confining pressure at the injection end of the core sample is greater than the injection pressure of the fluid, effectively ensuring that the pressure-driven fluid is injected into the core sample at a high pressure, forming the necessary pressure difference inside and outside the core sample to achieve the core fracture condition, and truly restoring the pressure-driven process of the rock in the reservoir, and realizing the accurate simulation of the pressure-driven crack expansion law and the pressure-driven enhanced recovery effect.

[0077] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", "connect", and "fix" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood by specific circumstances.

[0078] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A two-stage core pressure drive experimental device based on CT scanning, characterized in that: It includes a fluid supply system, a temperature control system, a two-stage pressure-controlled core holder, a pressure loading system, a CT scanner and a metering device; The fluid supply system comprises a constant speed and constant pressure pump and three intermediate containers, the three intermediate containers are respectively a formation water intermediate container, an oil intermediate container and a pressure drive liquid intermediate container, the constant speed and constant pressure pump is connected to the high-pressure injection pipelines of the formation water intermediate container, the oil intermediate container and the pressure drive liquid intermediate container respectively through a four-way valve, and is used to inject fluid into the two-stage pressure controlled core clamp; the outflow ends of the formation water intermediate container, the oil intermediate container and the pressure drive liquid intermediate container are all connected to high-pressure liquid outlet pipelines, each high-pressure liquid outlet pipeline is connected to the injection pipeline arranged at the injection end of the two-stage pressure controlled core clamp through a six-way valve, the outflow end of the two-stage pressure controlled core clamp is connected to the metering device through a discharge pipeline, and the discharge pipeline is provided with a back pressure valve and an outlet control valve; The two-stage pressure-controlled core clamp is used to clamp the core sample during pressure displacement; the CT scanner is matched with the two-stage pressure-controlled core clamp to monitor the fracture morphology of the core sample in real time during the pressure displacement process; The two-stage pressure-controlled core holder and the three intermediate containers in the fluid supply system are respectively connected to a temperature control system for controlling the temperature of the fluid injected into the core sample; The pressure loading system includes a confining pressure servo pump and a manual high-pressure pump, wherein the confining pressure servo pump is connected to the two-section pressure-controlled core clamp through a front-end pressure injection pipeline, and is used to apply confining pressure to the front end of the core sample in the two-section pressure-controlled core clamp; the manual high-pressure pump is connected to the two-section pressure-controlled core clamp through a rear-end pressure injection pipeline, and is used to apply confining pressure to the rear end of the core sample in the two-section pressure-controlled core clamp; a three-way valve is arranged on the rear-end pressure injection pipeline, and the back-pressure valve is connected to the three-way valve through a back-pressure valve connecting pipeline.

2. The two-stage core pressure drive experimental device based on CT scanning according to claim 1 is characterized in that: The two-stage pressure-controlled core clamp comprises a barrel, a front sealing cover and a rear sealing cover. The barrel has a core sample built in it, and the two ends of the core sample are sealed by the front sealing cover and the rear sealing cover respectively; one end of the core sample is provided with a water injection hole, and the other end is provided with a water outlet hole. The core experimental jacket is provided with a rubber sleeve, and the rubber sleeve is provided with a partition device to separate the annular cavity surrounded by the rubber sleeve and the inner wall of the barrel of the two-stage pressure-controlled core clamp into a front cavity and a rear cavity; The outer wall of the cylinder is provided with a front confining pressure injection hole and a rear confining pressure injection hole; the front confining pressure injection hole is connected to the front cavity and connected to the front pressure injection pipeline; the rear confining pressure injection hole is connected to the rear cavity and connected to the rear pressure injection pipeline; The front sealing cover and the rear sealing cover are both provided with insertion holes; the outflow end of the injection pipe passes through the insertion hole and the water injection hole of the front sealing cover in sequence, and is inserted into the core sample to inject fluid into the core sample; The inflow end of the discharge pipe passes through the insertion hole and the water outlet hole of the rear sealing cover in sequence, and is inserted into the core sample to discharge the fluid in the core sample.

3. The two-stage core pressure drive experimental device based on CT scanning according to claim 2 is characterized in that: The six-way valve is provided with an injection pressure gauge; the front end pressure injection pipeline is provided with a front end confining pressure pressure gauge near the confining pressure servo pump; the rear end pressure injection pipeline is provided with a rear end confining pressure pressure gauge near the manual high-pressure pump.

4. The two-stage core pressure drive experimental device based on CT scanning according to claim 3 is characterized in that: The temperature control system is configured as a constant temperature box.

5. The two-stage core pressure drive experimental device based on CT scanning according to claim 3 is characterized in that: The confining pressure servo pump is connected to an injection pressure gauge to obtain the injection pressure of the fluid in the core sample in real time.

6. A two-stage core pressure drive experimental method based on CT scanning, characterized in that: The two-stage core pressure drive experimental device based on CT scanning as claimed in claim 4 is used to measure the pressure drive fracture pressure and crack morphology of the core sample based on the constant speed method, including the following steps: Step 1, prepare a core sample, drill a circular groove for simulating fracturing perforation at the center of the side wall of the core sample, clean the surface of the core sample, dry it, evacuate it, saturate it with formation water, and then place it in a two-stage pressure-controlled core holder; Step 2, injecting formation water, oil and pressure-driven fluid into the formation water intermediate container, the oil intermediate container and the pressure-driven fluid intermediate container respectively; Step 3, according to the preset front end confining pressure value, the confining pressure servo pump is controlled to apply confining pressure to the front end cavity of the two-stage pressure controlled core clamp, and according to the preset rear end confining pressure value, the manual high-pressure pump is controlled to apply confining pressure to the rear end cavity of the two-stage pressure controlled core clamp. At this time, the front end confining pressure value is equal to the rear end confining pressure value. At the same time, the temperature control system is controlled according to the formation temperature of the reservoir to be simulated, so that the fluid temperature in the intermediate container and the temperature of the core sample in the two-stage pressure controlled core clamp are constant at the formation temperature, and then the four-way valve, the six-way valve and the outlet control valve are adjusted, and the formation water in the formation water intermediate container is injected into the two-stage pressure controlled core clamp by a constant temperature and pressure pump, and the core sample is displaced by formation water, and the formation water injection pressure is obtained according to the indication of the injection pressure gauge, and the water output of the discharge pipe is measured by a metering device. After the formation water injection pressure and the water output are stable, the initial water permeability of the core is calculated; Step 4, adjusting the four-way valve and the six-way valve, controlling the constant speed and constant pressure pump to inject the oil in the oil intermediate container into the two-stage pressure-controlled core holder at a preset flow rate, displacing the core sample with oil, and measuring the water output of the discharge pipe with a metering device. When the core sample reaches the initial oil-containing state, the water output of the discharge pipe no longer increases. The core sample in the initial oil-containing state is CT scanned with a CT scanner to obtain a three-dimensional core data body corresponding to the initial oil-containing state and construct a three-dimensional digital core to obtain the microscopic pore structure of the core sample in the initial oil-containing state; Step 5, control the confining pressure servo pump and the manual high-pressure pump, apply the rear confining pressure to the rear cavity of the two-stage pressure-controlled core clamp by the manual high-pressure pump according to the formation pressure of the reservoir to be simulated, and apply the front confining pressure to the front cavity of the two-stage pressure-controlled core clamp by the confining pressure servo pump, so that the front confining pressure is greater than the rear confining pressure, adjust the four-way valve and the six-way valve, control the constant speed constant pressure pump so that the injection pressure of the fluid in the two-stage pressure-controlled core clamp gradually increases to the rear confining pressure value, continue to control the constant speed constant pressure pump to increase the injection pressure of the fluid in the two-stage pressure-controlled core clamp and ensure that the front confining pressure is always greater than the injection pressure of the fluid in the two-stage pressure-controlled core clamp during the fluid injection process, inject the pressure-driven fluid in the pressure-driven fluid intermediate container into the two-stage pressure-controlled core clamp, use the pressure-driven fluid to displace the core sample, observe the injection pressure gauge, and use the metering device to measure the oil output of the discharge pipeline to obtain the pressure-driven fluid injection pressure and recovery rate change curve over time; Step 6, after the injection pressure indication is stable, the constant temperature and constant pressure pump is controlled to gradually increase the injection speed of the pressure-driven fluid according to the preset pressure gradient, and the injection pressure change curve under different flow rates is obtained. The indication of the injection pressure gauge is observed until a sudden pressure drop occurs. At this time, cracks are generated in the core sample. The critical fracture pressure of the pressure-driven fluid is determined according to the indication of the injection pressure gauge when the pressure drops suddenly. The core sample after the cracks are generated is scanned by a CT scanner to obtain a three-dimensional data body of the core after the cracks are generated and a three-dimensional digital core is constructed to obtain the microscopic pore structure and crack morphology of the core sample after the cracks are generated.

7. A two-stage core pressure drive experimental method based on CT scanning, characterized in that: The two-stage core pressure drive experimental device based on CT scanning as claimed in claim 4 is used to simulate the pressure drive fracture expansion law and the pressure drive enhanced oil recovery effect based on the constant pressure method, including the following steps: Step 1, prepare two identical core samples, drill a circular groove for simulating fracturing perforation at the center of the side wall of the core sample, clean the surface of the core sample, dry it, evacuate it, saturate it with formation water, and then randomly select one core sample and place it in a two-stage pressure-controlled core holder; Step 2, injecting formation water, oil and pressure-driven fluid into the formation water intermediate container, the oil intermediate container and the pressure-driven fluid intermediate container respectively; Step 3, according to the preset front end confining pressure value, the confining pressure servo pump is controlled to apply confining pressure to the front end cavity of the two-stage pressure controlled core clamp, and according to the preset rear end confining pressure value, the manual high-pressure pump is controlled to apply confining pressure to the rear end cavity of the two-stage pressure controlled core clamp. At this time, the front end confining pressure value is equal to the rear end confining pressure value. At the same time, the temperature control system is controlled according to the formation temperature of the reservoir to be simulated, so that the fluid temperature in the intermediate container and the temperature of the core sample in the two-stage pressure controlled core clamp are constant at the formation temperature, and then the four-way valve, the six-way valve and the outlet control valve are adjusted, and the formation water in the formation water intermediate container is injected into the two-stage pressure controlled core clamp by a constant temperature and pressure pump, and the core sample is displaced by formation water, and the formation water injection pressure is obtained according to the indication of the injection pressure gauge, and the water output of the discharge pipe is measured by a metering device. After the formation water injection pressure and the water output are stable, the initial water permeability of the core is calculated; Step 4, adjusting the four-way valve and the six-way valve, controlling the constant speed and constant pressure pump to inject the oil in the oil intermediate container into the two-stage pressure-controlled core holder at a preset flow rate, displacing the core sample with oil, and measuring the water output of the discharge pipe with a metering device. When the core sample reaches the initial oil-containing state, the water output of the discharge pipe no longer increases. The core sample in the initial oil-containing state is CT scanned with a CT scanner to obtain a three-dimensional core data body corresponding to the initial oil-containing state and construct a three-dimensional digital core to obtain the microscopic pore structure of the core sample in the initial oil-containing state; Step 5, control the confining pressure servo pump and the manual high-pressure pump, apply the rear confining pressure to the rear cavity of the two-stage pressure-controlled core clamp by the manual high-pressure pump according to the formation pressure of the reservoir to be simulated, and apply the front confining pressure to the front cavity of the two-stage pressure-controlled core clamp by the confining pressure servo pump, so that the front confining pressure is greater than the rear confining pressure, adjust the four-way valve and the six-way valve, control the constant speed constant pressure pump so that the injection pressure of the fluid in the two-stage pressure-controlled core clamp gradually increases to the rear confining pressure value, continue to control the constant speed constant pressure pump to increase the injection pressure of the fluid in the two-stage pressure-controlled core clamp and ensure that the front confining pressure is always greater than the injection pressure of the fluid in the two-stage pressure-controlled core clamp during the fluid injection process, inject the pressure-driven fluid in the pressure-driven fluid intermediate container into the two-stage pressure-controlled core clamp, use the pressure-driven fluid to displace the core sample, observe the injection pressure gauge, and use the metering device to measure the oil output of the discharge pipeline to obtain the pressure-driven fluid injection pressure and recovery rate change curve over time; Step 6, after the injection pressure indication is stable, the constant temperature and constant pressure pump is controlled to gradually increase the injection speed of the pressure-driven fluid according to the preset pressure gradient, and the injection pressure change curves under different flow rates are obtained, and the indication of the injection pressure gauge is observed until a sudden pressure drop occurs. At this time, cracks are generated in the core sample, and the pressure-driven critical fracture pressure is determined according to the indication of the injection pressure gauge when the pressure drops suddenly, and the core sample after the cracks are generated is CT scanned by a CT scanner to obtain a three-dimensional data body of the core after the cracks are generated and construct a three-dimensional digital core to obtain the microscopic pore structure and crack morphology of the core sample after the cracks are generated; Step 7, using another core sample to replace the core sample in the two-stage pressure-controlled core holder, repeating steps 3 and 4, and obtaining the microscopic pore structure of the replaced core sample in the initial oil-containing state; Step 8, according to the indication of the injection pressure gauge, control the confining pressure servo pump to apply a confining pressure value higher than the injection pressure of the fluid in the two-stage pressure-controlled core holder to the cavity at the front end of the two-stage pressure-controlled core holder, set a constant temperature and pressure pump according to the pressure-driven critical fracture pressure determined in step 6, control the constant temperature and pressure pump to inject the pressure-driven fluid in the pressure-driven fluid intermediate container into the core under the pressure-driven critical fracture pressure condition, use the pressure-driven fluid to displace the core sample, and use a metering device to measure the oil output of the discharge pipeline and the pressure-driven fluid flow rate; Step 9, after a preset amount of pressure-driven fluid is injected into the two-stage pressure-controlled core holder, a CT scanner is used to perform a CT scan on the core sample after the pressure-driven fluid is injected to obtain a three-dimensional digital core and pore structure corresponding to the core sample in the current state; Step 10, after the injection of the pressure-driven fluid is completed, adjust the four-way valve and the six-way valve, control the constant temperature and pressure pump to inject the formation water in the formation water intermediate container into the two-stage pressure-controlled core clamp at a constant speed, and use the formation water to displace the core sample until the oil output measured by the metering device no longer changes, then end the formation water displacement of the core sample, and use the CT scanner to perform a CT scan on the core sample again to obtain the three-dimensional digital core and pore structure corresponding to the core sample after the pressure drive is completed.

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